The username can be retrieved in Windows with %username%, but there is no environment variable for "group" that can be used to dynamically set the navigation path.
You will have to fiddle around with creating an environment variable for the group while a user is "switched to" (done by the Object Spawner) that you can use in the metadata.
This environment variable can be set in the WorkspaceServer_usermods.bat file and used in the Workspace Server definition in SAS metadata. I just verified that by testing it on UNIX:
1. create environment variable in WorkspaceServer_usermods.sh:
export HOMEPATH=/somepath
2. set the navigation root in SAS metadata to
$HOMEPATH/$USER
and I ended up in /somepath/myusername when I opened the workspace server in EG.